NASA's Hubble Space Telescope has discovered carbon dioxide in the atmosphere of a hot jupiter planet, a crucial step towards finding the chemical tracers of extraterrestrial life in other solar systems.

"In the terrestrial planets of our Solar System, carbon dioxide plays a crucial role for the stability of climate, says Giovanna Tinetti from University College London. “On Earth, carbon dioxide is one of the ingredients of the photosynthesis and a key element for the carbon cycle. Our observations represent a great opportunity to understand the role of carbon dioxide in the atmospheres of hot Jupiter-type planets."

Although the Jupiter-sized planet HD 189733b is far too hot for life, the Hubble observations demonstrate that the basic chemistry for life can be measured on planets orbiting other stars. Previous observations of HD 189733b by Hubble and the Spitzer Space Telescope identified water vapour and methane in the planet's atmosphere. "This is exciting because Hubble is allowing us to see molecules that probe the conditions, chemistry, and composition of atmospheres on other planets," says team leader Mark Swain of NASA's Jet Propulsion Laboratory. "Thanks to Hubble we're entering an era where we are rapidly going to expand the number of molecules we know about on other planets."

Artist impression of HD 189733b, which lies 63 light years away. Astronomers have now detected carbon dioxide and carbon monoxide in the planet’s atmosphere, in addition to methane and water vapour. Image: NASA, ESA, and A. Feild (STScI).

Swain and colleagues used Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to study infrared light emitted from the planet as it routinely passed in front of and behind its parent star once every 2.2 days. This afforded the astronomers the opportunity to subtract the light of the star alone (when the planet is blocked) from that of the star and planet together prior to eclipse, therefore isolating the emission of the planet alone and arriving at a chemical analysis of its ‘day-side’ atmosphere. "In this way, we are using the eclipse of the planet behind the star to probe the planet's day side, which contains the hottest portions of its atmosphere," says team member Guatam Vasisht of JPL. "We are starting to find the molecules and to figure out how many there are to see the changes between the day side and the night side."

The observations revealed the spectral fingerprints of not only carbon dioxide, but also carbon monoxide. This is the first time a near-infrared emission spectrum has been obtained for an exoplanet. "The carbon dioxide is kind of the main focus of the excitement, because that is a molecule that under the right circumstances could have a connection to biological activity as it does on Earth," says Swain. "The very fact that we're able to detect it, and estimate its abundance, is significant for the long-term effort of characterising planets both to find out what they're made of and to find out if they could be a possible host for life."

This successful demonstration of looking at near-infrared light
emitted from a planet is very encouraging for astronomers planning to use NASA's James Webb Space Telescope (JWST) when it is launched in 2013, since these biomarkers are best seen at near-infrared wavelengths and furthermore, astronomers hope to detect and study Earthlike planets with it. Meanwhile, the international research team plans to search for more bio-molecules in the atmospheres of other exoplanets, as well as trying to increase the number of molecules detected in exoplanet atmospheres, which could yield new information regarding the weather on these distant worlds.

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